A complementary approach is to model the modes of variation present in the spectral line profile independently of their temporal behaviour, as explored by YARARA. While a planet produces a shift of all spectral lines, the stellar activity affects the shape of the spectral lines. The FourIEr phase SpecTrum Analysis (FIESTA, a.k.a. \(\Phi \) ESTA) [1]}, [2]} uses the real and imaginary parts of the Fourier transform of the line profile to disentangle apparent RV shift due to spectral line-profile variability from true Doppler shift. Concurrently, [3]} also developed machine learning techniques such as linear regression and neural networks to separate activity signals from true center-of-mass RV shifts using the average spectral line-shape variations, without any time-domain information. Another recent investigation [4]} employs Doppler imaging to simultaneously model the activity-induced distortions and the planet-induced shifts in the line profiles. It is challenging to differentiate between the effects induced by a planet and stellar activity, as the latter has varied effects on individual spectral lines [5]}.
